Deep Foundations

What Are Deep Foundations?

Deep foundations transfer loads through weak near-surface soils down to stronger strata or rock at depth. They are essential when shallow options cannot meet bearing capacity, settlement, or lateral resistance requirements. Typical applications include high-rise buildings, bridges, waterfront structures, tanks on soft ground, and structures in seismic or liquefiable environments.

This guide covers pile and drilled shaft types, selection criteria, axial and lateral design, group interaction, construction QA/QC, durability issues (corrosion, scour, negative skin friction), and a practical workflow linked to Site Characterization, Geotechnical Modeling, and Design Software.

Deep foundations succeed when load path, ground model, and construction method are aligned—and verified in the field.

Types of Deep Foundations

Deep systems vary by installation method, material, and how they mobilize resistance (end bearing vs. shaft friction). Choice depends on subsurface conditions, loads, noise/vibration limits, and access.

  • Driven Piles: Steel H-piles, open/closed-ended pipe piles, precast prestressed concrete piles, and timber. Installed by impact or vibratory hammers; provide quality via driving records and dynamic testing.
  • Drilled Shafts (Bored Piles/Caissons): Cast-in-place concrete elements constructed with casing or slurry; excellent for large axial and lateral loads with minimal vibration.
  • Micropiles: Small-diameter, high-capacity, drilled and grouted piles; ideal for restricted access, seismic retrofits, and underpinning.
  • Augercast/ACIP Piles: Constructed using continuous flight augers; low vibration option for urban sites.
  • Composite/Hybrid Systems: Pile-rafts or combined pile-mats that share load between shallow and deep elements.

Related topics

For basics on pile systems and alternatives, see Pile Foundations and complementary Shallow Foundations.

When to Choose Deep Foundations

Deep solutions are justified when shallow footings cannot achieve performance targets economically or safely. Key drivers include:

  • Weak/Compressible Near-Surface Soils: Thick soft clays/organic deposits requiring excessive mat thickness or intolerable Settlement Analysis.
  • High Loads & Uplift: Tall towers and bridges with significant overturning and tension demands.
  • Groundwater/Scour/Seismic Risks: Liquefaction potential (Liquefaction), scour at waterways, or lateral spreading hazards.
  • Right-of-Way Constraints: Footprint or utility conflicts making mats impractical.

Did you know?

For critical infrastructure, load testing can economically unlock higher design capacities than correlations alone—often reducing pile counts and total cost.

Axial Capacity: Principles & Checks

Axial resistance derives from end bearing at the tip and friction along the shaft. Designers estimate ultimate capacity using empirical correlations (SPT/CPT), static analysis, and confirm with load testing. Apply an appropriate Factor of Safety or resistance factors per the governing code and risk profile.

Static Axial Capacity (Concept)

\( Q_\text{ult} = q_p A_p + \sum f_s A_s \)
\(q_p\)Unit tip resistance
\(A_p\)Pile tip area
\(f_s\)Unit shaft friction by layer
\(A_s\)Shaft surface area by layer

In clays, undrained shear strength and adhesion factors govern; in sands, effective stress and lateral earth pressure around the pile control shaft friction, while tip resistance scales with relative density and confining stress. Settlement acceptance criteria (e.g., 5–10% of diameter for piles during load test) inform serviceability. For rock-socketed shafts, side resistance in the socket and tip bearing on rock are verified by construction records (socket roughness/cleanliness) and dedicated load tests.

Data to Parameters

Trace parameters to tests using a disciplined pipeline (see Geotechnical Data Analysis and Geotechnical Soil Testing).

Lateral, Uplift & Seismic Performance

Deep foundations resist lateral loads via soil–pile interaction. Designers use p–y curves, elastic continuum solutions, or 3D numerical models to capture stiffness and capacity, and check uplift using shaft friction and, if available, tip suction in clays. Seismic design addresses kinematic interaction, inertial loads, and liquefaction-induced demands.

Lateral Response (Concept)

Soil springs: \(p = k(y)\,y\), integrated along pile length → moment & shear envelopes
\(p\)Soil reaction per unit length
\(y\)Lateral deflection
  • Liquefaction & Lateral Spreading: Evaluate triggering and ground deformation; consider ground improvement or extended embedment (see Liquefaction).
  • Site Class & Hazard: Use stable national sources like USGS for seismic hazard; verify VS30 via Seismic Testing.
  • Uplift: Use conservative friction factors; verify with tension load testing where critical.

Pile Groups, Spacing & Block Behavior

When piles act in groups, the composite response can be less than the sum of individual capacities due to stress overlap and block action. Spacing typically ranges from 2.5–3.0 diameters (minimum) upward, depending on soil type and installation method. Group efficiency, settlement compatibility, and load sharing with caps/rafts require attention.

  • Group Efficiency: Empirical or analytical reduction factors account for shadowing and limited mobilization of friction.
  • Cap Stiffness: Stiff pile caps distribute loads and control differential movement—coordinate with structural modeling.
  • Negative Skin Friction (NSF): Consolidation of surrounding soils can add downdrag; isolate with sleeves or coatings and design for combined structural + downdrag demands.

Example: Bridge Pier in Soft Clay

A driven pile group in normally consolidated clay was spaced at 3D with a thick cap. Consolidation settlement of surrounding clay induced downdrag; design incorporated bitumen sleeves in the upper 6 m and a neutral plane analysis. Lateral demands were satisfied with p–y modeling and verified through restrike testing. The solution reduced predicted long-term pier settlement by 35% relative to unsleeved piles.

Neutral Plane Concept

Locate depth where pile and soil settle equally—above: downdrag; below: positive friction.

Construction Methods, Testing & QA/QC

Construction influences capacity as much as design math. Specify procedures and acceptance criteria so installed elements match assumptions.

  • Driven Piles: Hammer selection, driving criteria/blow count, driving records, and restrike (WEAP/GRLWEAP). Dynamic measurements (PDA) estimate capacity and stresses; static load tests provide definitive confirmation.
  • Drilled Shafts: Slurry type/parameters, hole cleanliness, base inspection (camera/sonic), casing, tremie concrete placement, and cross-hole sonic logging (CSL) or thermal integrity profiling (TIP).
  • Micropiles/ACIP: Grout pressure/volume monitoring, reinforcement, and integrity testing.
  • Load Testing: Compression, tension, and lateral tests with instrumentation (strain gauges, telltales) to define load–settlement response and stiffness for serviceability checks.

Important

Never rely solely on correlations in complex ground. Calibrating design with project-specific load tests can save cost by reducing pile count or length.

Durability, Scour, Corrosion & Downdrag

Long-term performance hinges on anticipating environmental actions:

  • Scour & Erosion: For waterways, determine design scour depths and embed piles accordingly; consult durable references like FHWA and USACE.
  • Corrosion & Sulfates: Steel loss allowances, coatings, cathodic protection; concrete cover and mix design for sulfate/chloride environments.
  • Negative Skin Friction: Manage with sleeves, coatings, preloading, or settlement-tolerant connections; revisit where fills or surcharge are planned (see Soil Consolidation).
  • Groundwater Control: Consider buoyancy, seepage, and construction dewatering impacts (see Groundwater).

Design Workflow: From Data to Decision

A repeatable workflow builds confidence, reduces rework, and streamlines reviews. Tie each step to evidence and document assumptions.

  • 1) Site Characterization: Logs, CPT, geophysics, groundwater, and hazards mapping (see Site Characterization; background via USGS).
  • 2) Data Analysis: Clean, normalize, and parameterize (Geotechnical Data Analysis).
  • 3) Preliminary Sizing: Static estimates for axial/lateral; evaluate alternatives (driven vs. drilled vs. micropile) using Geotechnical Modeling.
  • 4) Load Testing Plan: Select test elements/locations to bound key uncertainties; refine design resistances.
  • 5) Detailing & Specs: Construction controls, acceptance criteria, integrity testing, and monitoring.
  • 6) Reporting: Provide load–settlement curves, capacity derivations, and construction recommendations per Geotechnical Reporting.

Case Snapshot: Coastal Pier Upgrade

Existing timber piles were replaced by steel pipe piles driven to dense sand over weathered rock. Preliminary design used CPT-based static methods; PDA during driving confirmed stresses and capacities. A static compression test validated higher resistance, enabling a 12% reduction in pile count. Scour analysis (per FHWA) increased embedment, and corrosion protection combined coatings with sacrificial thickness. The finalized design balanced performance and lifecycle cost.

Design Logic

Data → Parameters → Preliminary Sizing → Test & Calibrate → Optimize → Detail → Monitor

FAQs: Deep Foundations

Driven piles or drilled shafts—how do I choose?

Consider access, noise/vibration limits, groundwater management, required capacities, and soil profile. Driven piles excel in production rate and QC via driving records; drilled shafts suit high capacities with low vibration in urban settings.

How many load tests do I need?

Enough to bound key uncertainties (soil variability, installation method) and calibrate design resistances. Critical structures benefit from at least one compression test and, where relevant, tension or lateral tests.

How do liquefaction and lateral spreading affect deep foundations?

They reduce lateral/axial resistance and can impose kinematic demands. Evaluate triggering, ground deformation, and consider ground improvement or deeper embedment (see Liquefaction).

What about uplift?

Uplift capacity comes from shaft friction and, in clays, potential suction. Verify with tension load testing when uplift governs. Detailing of pile caps and connections is critical.

Where can I find stable references?

National agencies provide durable resources: FHWA (drilled shafts, driven piles), USACE (foundations and waterfront structures), and USGS (seismic, geology, and hazards).

Conclusion

Deep foundations provide reliable load transfer when near-surface soils cannot. Success relies on a clear subsurface model, fit-for-purpose element type, calibrated axial/lateral resistance, and rigorous construction QA/QC. Anchor context to durable sources like FHWA, USACE, and USGS. For connected topics, explore Pile Foundations, Bearing Capacity, Settlement Analysis, Groundwater, and Geotechnical Reporting. With a traceable, test-calibrated workflow, deep foundations can be optimized for performance, constructability, and cost.

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